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What is differance between the followings terms??

1-PWHT
2-STRESS RELIEVING

Moreover, why we control hardness level of welded joints if equipmnets are PWHT.How hardness level practillay controlled during PWHT??

The purpose of both the terms PWHT or Stress Relieving is the same that is to remove residual stress in addition to reducing the hardness in heat affected zone. The process is also the same for performing the both. But the terms PWHT (Post Weld Heat Treatment) is specifically linked with the residual stresses caused by welding and stress relieving encompasses all the residual stresses that may result during fabrication operations of a vessel including machining, rolling, forming etc.

Not all the time, hardness of the weld comes under acceptable limit by PWHT if welding parameters were not controlled during welding job. It becomes extremely critical when you are dealing with low alloy steels in P.No. 3, P.No. 5A, B, C materials.
Following the right method prescribed in code would practically bring the hardness level within acceptable range but care must be exercised during welding operation also.

Hope this would help.

Thanks Asfaq.Please describe why hardness is controlled during PWHT. (e.g if someone say control hardness level 200 to 220 BHN then what is reason behind it??.How can we calculate hardness level limiting value??).

PWHT is required to reduce the hardness also as induced by the welding process especially in higher thickness (that's why PWHT is recommended above a certain thickness).
Hardness limits are specified in the light of applicable code as the final hardness at which pressure component has to operate must be within the acceptable limits. The component must have good ductility characteristics also which are induced by PWHT operation.
Hardness level limiting values are provided by the applicable code. Do not calculate.

Dear Rana,
When a metal gets harder during welding, it becomes more brittle which is unacceptable. Preheat, inter-pass temp controls and post heat all are designed to reduce the potential for brittleness. Acceptable hardness levels are defined by the code applicable in your case.

Thanks.